Purple-colored pigments on the basis of colloidally distributed elemental gold in an oxidic or silicate matrix, such as glass frits, are known. Purple pigments of this kind can be produced, for example, by the reductive precipitation of gold from an aqueous gold salt solution in the presence of glass frits or a metal oxide or by the precipitation of Cassius purple, mixing of the latter with glass frits or a metal oxide, presintering at 600 to 800.degree. C. and subsequent milling of the sintered material. According to DE-OS 44 11 104 purple pigments can be produced by the bringing of a finely powdered silicate or oxidic substrate material into contact with one or more gold compounds and subsequent thermal treatment of the mixture at 150 to 300.degree. C. It is common to the purple pigments produced according to the aforementioned process principles that they are not spherical, but exhibit the irregular surface structure known from the oxidic or silicate matrix, such as glass frits, used. According to DE-PS 32 29 837 plate-like particles coated with metal oxides can also be converted into cerise pigments by tempering at 600.degree. C. after treatment with an organic gold resinate solution. The plate-like structure is retained in said process.
Pigments as finely dispersed as possible are of interest whenever the materials to be pigmented represent very thin decorative layers, films or fibers. In order to achieve the desired fine dispersion in the case of inorganic pigments, laborious milling processes are often required. The technical complexity increases with decreasing mean grain diameter, and pigments according to the preamble with a mean grain diameter of about 1 .mu.m or less are, for practical purposes, no longer obtainable by the conventional size-reduction techniques. There is in addition the risk of the color of the pigment being modified by rubbing from milling auxiliaries employed and the mills used. The pigments obtained by conventional methods have an irregular shape with many broken edges. Such pigments, even if they possess a high degree of fineness and a narrow grain spectrum, cause problems in the spinning of materials dyed with them. With melt spinning, for example, corrosion phenomena on the spinning nozzles and/or other defects, such as clogging of the spinning nozzles, are found. Corrosion phenomena of this kind have limited the use of conventional pigments of non-spherical shape to date.
In order to avoid application problems due to the irregular shape of the pigments, there is a requirement for pigments with as spherical a habit as possible, which should exhibit, in the event of a very small mean grain diameter, for example one in the range from 0.1 to 10 .mu.m, and preferably, in addition, a very narrow grain spectrum.
Metallic and ceramic spherical particles in the few microns to sub-micron range are available by means of the so-called spray-pyrolysis technique. In this technique an aerosol consisting of a solution or suspension of materials from which the fine powder to be produced can be formed is introduced into a heated reactor, wherein the solvent or suspension agent is evaporated and solid residue particles are formed, which for their part can be converted into the desired product by a decomposition and/or another chemical reaction. Spherical glass powders with a particle diameter of between 0.1 and 5 .mu.m are obtainable according to JP-A 08091874 by the injection of fine drops of one or more solutions of precursors of the metal oxides contained in the glass into a flame at 800 to 1800.degree. C. and subsequent rapid cooling. According to EP-B 0 371 211 there can be obtained, by injection of an aqueous solution containing various metal salts into a pyrolysis reactor, with hydrogen being used as spray gas, ceramic powders consisting of spherical particles with a particle diameter in the range from 0.1 to 3 .mu.m. According to EP-A 0 681 989 it is also possible to introduce the aerosol into an independently operated detonating gas flame, wherein a lower flame temperature is adjustable and hence other modifications are obtainable. A further modifying is taught in DE-OS 43 07 333, according to which an aqueous solution of the metal compounds which is dispersed in an organic phase is fed to the pyrolysis reactor in aerosol form. According to J. Chem. Soc. Japan (1987), Vol. 12, pp. 2293-2300 it is also possible to pass an aerosol of an aqueous solution of a noble metal salt into a hydrogen-oxygen flame (detonating gas flame), wherein noble metal powders are obtained. While the aerosol pyrolysis is carried out for the most part in so-called flame pyrolysis reactors, it is also possible to carry out the pyrolysis in a furnace heated from the outside.
None of the processes known to date using aerosol pyrolysis is directed to the production of pigments on the basis of an oxidic or silicate substrate material with noble metal particles distributed extremely finely on/or in the particles. The object of the present invention is consequently the provision of such pigments consisting of substantially spherical pigment particles, also described simply as spherical pigments below.